One thing you can rely on with Canon cameras is the quality of the components and the way they're put together. Build quality is simply superb, and everything works quickly and smoothly. Of particular note is the LCD monitor, which is one of the best I've seen on a compact camera. It measures three inches diagonally with a resolution of 230k dots, and is superbly bright and sharp, but it is the viewing angle that really impresses. It can be viewed from almost any angle; if you can see the screen, you can see what's on it. This is great for shots where you have to hold the camera at an unusual angle, such as shooting over crowds.
Turning to the control layout, the designer has gone for a motif of triangles and zig-zags. The control buttons on the back of the camera are large, but are arranged in a single curved strip down the right edge of the body. Unfortunately this leaves very little room to grip the camera, and despite the slight flaring of the right end of the front panel, the camera is quite hard to hold securely without accidentally pressing buttons with your thumb. Mode selection is via a rotating ring around the D-pad, and this too is very easy to jog while shooting. Some other recent Canon ultra-compacts have had issues with ergonomics. It's all very well making a camera look distinctive and interesting, but this should not be at the expense of usability. Canon could take a few lessons from Samsung and Panasonic in this department.
The 870 IS is a relatively straightforward point-and-shoot camera, and as such it isn't overburdened with features, but those it does have work well. It has Canon's impressive optical image stabilisation system, which is as good as ever, as well as an effective face detection system. This is used in the 870's one unusual feature, the Face Self-timer function. This is a clever and genuinely useful feature for social photography. Usually if you want to include yourself in a group shot, you have to set the self timer then quickly run around and join in the group, giving you just a couple of seconds to compose yourself and smile for the shot. The Face Self-timer mode detects the faces of your group, and then waits until it detects that a new face is added. It waits a further two seconds and then takes three shots in a row. It's simple, ingenious and works well.